Armor pieces and layering:Here's a list of what parts a armor could consist and what the elements of those parts could be.
Helmet:-Head shielding (the main helmet)
-Face plate (can be taken off or opened on closed helmets or added to open helmets)
Cuirass:-Body armor (front and back part)
-Neck shield
-Simple shoulder shields
-Lower body (hips and crotch overlapping) shielding
Shoulder:-Shoulder shielding
Upper arm:-Shoulder plating
-Upper arm plating
Forearm:-Forearm plating
-Elbow protector
Hands:-Hand protection
-Wrist shield
Hips and upper legs:-Hips and crotch armoring
-Thighs armor
Lower legs:-Lower leg armoring
-Knee protector
Feet:-Upper foot shielding
-Soles (sandals for example ONLY protect the soles)
-Ankle and lower leg protection (boots)
Tail armor (Beast races only):-Tail armoring
This would be the maximum of separate armor parts for a complete set aside pieces that can be removed or break off.
Some armor parts would actually be merged with each other for a few armor types so one piece takes more than one slot, in some cases the gloves and forearm shielding would be in one piece or both arm shields would be one. This would especially count for shoulder shields, some would actually be part of the cuirass or the upper arm shield.
If you'd like to combine certain armor parts it's either not possible (like using shoulder shields on a cuirass that has unremovable ones) or you'd have to take the armor apart to make a free slot.
In some armors types certain components would also be missing, best example are helmets that have no visor or face plate. Some upper leg armors for example wouldn't have protection for the hips and crotch as the fitting cuirass already covers those.
This can lead into certain armor piece combinations leaving holes in your protection. On the other hand certain pieces that where not meant for each other can inhibit your mobility as they block each other
BTW, yea beast races would have one more armor slot but I'm against the universal armoring system and instead want one where every body part is protected separately. That would mean having that extra tail slot simply means having one more place that needs protection.
As for layering clothing and armor it could go by their "size" and slot, the general separation would be:
Underclothing:-Shirts
-Light and short pants
-Socks
-Light gloves
-Cloth masks
-Headscarf
-Veil
...
Overclothing:-Jackets
-Thick shirts/pullovers
-Pants
-Scarfs
-Masks
-Glasses
...
Armors:Pretty much all armors...
Covering clothing:-Wide jackets
-Ponchos
-Cloaks
-Skirts/Kilts
-Capes
-Hats
-Hoods
...
Some armoring like chainmail however can be worn between under- and overclothing as well to hide it, perfect for assassins. Belts could be worn on any clothing layer but for holding items they make most sense on the outermost.
There might be a problem for certain armors and covering clothing, look at the daedric helmets for example, you'd hardly get a hood over that... tough it would look cool to put a hat on top of it
.
It could be possible to also wear a small non held shield on your forearm for archers to defend since they have both hands occupied.
Also all pieces of clothing should add some armoring value, after all clothes ARE protection of your environment. Thicker gloves generally count as armor since, aside finer ones, they are to protect your hands from getting hurt. This means when wearing thick gloves already you can't wear armored gloves on top but you can wear thin gloves below.
Also shoes count as armor, you can't wear boots over them. You can however wear socks under all shoes.
Not entirely sure on how the armor pieces should be split or merged though.